Generally lever locks contain a plurality of pivoted lever tumblers and a dead bolt, all of which are directly engaged by a rotating key. In such locks lever steps on a key bit engage the lever tumblers and a bolt step on the key bit engages a bolt talon for moving the bolt. A bolt stump projecting from the bolt travels on the tumblers between lever pockets through lever gatings, determining on the tumblers locking and unlocking of the lock. Such locks can be locked and unlocked only when the lever steps cause the gatings to be aligned, thus enabling the bolt stump to travel between the lever pockets.
Such prior art lever locks are pickable. The key held in hand by its bow end on the outside, directly contacts the bellies of the lever tumblers and the bolt talon inside the lock when the key is rotated. Thus, there is always available a continuous and clear passage from the outside of the lock through the keyhole and leading to the lever tumblers and bolt.
The lever tumblers and the bolt in conventional locks are independent of each other to a great extent. Every lever tumbler can be lifted or moved independently. The bolt can also be independently moved laterally up to a point where the bolt stump which is positioned within lever pockets of the tumblers, is prevented from further movement due to non-alignment of the gatings.
Operating with suitable picking tools through the keyhole, continuous pressure can be applied on the bolt talon while simultaneously lifting every lever tumbler separately so it is made to travel against the bolt stump. By this method the points of resistance for the individual lever tumbers can be analyzed and the lock successfully picked.
High security lever locks known in the prior art are provided with various additional security devices like detectors, protectors, detents, false notches, barrels and curtains, bullets, wards and guards, etc. These additional devices make the task of picking more difficult and time-consuming but not theoretically impossible.
Lever locks are lockable only by using their keys. This is also a drawback when dealing with emergency locking such as during holdups.
Thus there exists a need in the prior art for a lever lock that is theoretically unpickable and which can be locked without using its key.